Calculated Field Looker to extract certain part of page path

[Chris108]

I'm trying to aggregate sessions based on the input in a certain field which is then displayed in the URL. Since this wasn't set up properly in the Tag Manager I want to analyze the existing data by extracting the specific part of the URL from the page path. I've tried to do this with a calculated field using the following regex: 

REGEXP_EXTRACT(Page path, 'address=([^&]*)')

but I keep getting the error "Formula is invalid".

Not all Page paths contain "address" but I want to keep only the ones that do have address=xxx. Before address I have either a ? or & and after the input on address the url either ends or there is another ?. 
I don't have much experience with neither Looker Studio nor Regex so any help would be appreciated.

[sam8]

Where does Looker Studio tell you that this is invalid? I tested this out and it seems to be a perfectly valid formula.